1. Field of the Invention
This invention relates to viscosity stable chloroprene-sulfur copolymers.
2. Related Art
"Sulfur modified" neoprenes (so called, but such compositions are in fact copolymers of chloroprene and sulfur) have had and continue to have wide commercial success. The copolymers are prepared by polymerizing chloroprene in an aqueous emulsion in the presence of sulfur as widely described in the art, for example, Whitby, "Synthetic Rubber", 1954, page 770. The chloroprene is polymerized with sulfide linkages in the polymer chains. The product is a high molecular weight, high viscosity polymer, which would not be tractable in ordinary rubber milling and processing equipment.
Generally the product rubber is subjected to a procedure to cause scission of the long molecular chains at the sulfide linkages to reduce the molecular weight of the rubber and produce rubber of the desired viscosity. The process of chain scission has been called "plasticizing" at times, however, it is believed the more appropriate term is peptization.
Tetraethyl thiuram disulfide (TETDS) is the principal peptizing agent and it causes the desired scission of the polymer chains at the sulfide linkages, however, TETDS peptization has always been a delicate and troublesome procedure and various means have been devised to eliminate problems associated therewith.
One aspect of peptization with TETDS is the provision of a sufficient period of time for the peptization to occur, usually 4 to 20 hours. In addition, the peptized chloroprene/sulfur comonomers often exhibited viscosity instability on storage. The presence of a small amount of residual TETDS in the rubber after peptization, which serves as a processing aid, may continue to cause peptization. Dithiocarbamate is frequently added to speed peptization.
U.S. patent application B 324,505 (Trial Voluntary Protest Program) discloses that improved peptizations, i.e. good rate and stable product, can be achieved by utilizing the dithiocarbamate to TETDS at a substantially constant ratio regardless of the degree of peptization desired.
However, the difficulties so long associated with TETDS and the real lack of solution thereto has given impetus to development of alternate peptization methods avoiding the effective but troublesome TETDS. U.S. Pat. No. 3,920,623 discloses a peptization system of benzothiazole sulfenamide and dialkyl xanthogen disulfides, alkyl xanthic acid salts or dialkyl-dithiocarbonic acid salts, carried out at 10.degree.-50.degree. C. for 1 to 24 hours, wherein TETDS is avoided as detracting from the properties of the "Vulcanizates".
Another alternative method of peptization is disclosed in U.S. Pat. No. 4,303,765 where chloroprene-sulfur copolymers are peptized with specified polysulfides.
U.S. Pat. No. 3,984,609 discloses sulfur modified polychloroprene, obtained by polymerizing chloroprene in the presence of sulfur and either a dialkyl xanthogen disulfide or iodoform, can be instantaneously peptized with a class of thiol compounds including mercaptobenzothiazole or its sodium salt at pH of at least 9.
The present invention has established that mercaptobenzothiazole or its soluble salts have no effect on chain scission (peptization) of chloroprene-sulfur copolymer, however, in combination with TETDS, instantaneous peptization is obtained, in a highly reproducible manner to produce a rubber with excellent Mooney viscosity stability and some improved properties in cured products.